Various approaches have been taken to protect telecommunications transmission lines and associated telecommunications circuits and equipment coupled to the lines from overvoltage and overcurrent conditions that may occur on the incoming lines. Such conditions may occur as a result of a variety of conditions including lightning strikes and AC power line related fault events.
The earliest techniques for providing such protection included spark gap protectors and carbon block overvoltage protectors. Various protector devices are available which include both primary and secondary protection stages. For example, gas tube protectors have been coupled with an air gap back-up protection in a protector device so that if the primary surge arrestor, the gas tube protector, were to fail, the back-up device takes over thereby generally protecting the communications circuits from transient overvoltage events.
Other design approaches have utilized solid state voltage clamping devices, such as diodes, in combination with gas discharge type overvoltage arrestors. The solid state devices may be configured to provide purely back-up protection under failure conditions of the gas discharge tube or may be configured as true hybrid protection stages in the device. The hybrid application typically utilizes the solid state devices to benefit from their faster response time in clamping voltage to provide protection until the gas discharge tube switches while continuing to use the gas discharge tube to handle the large current flows sometimes encountered during overvoltage events on telephone communications lines. Examples of devices providing line protection through the use of both solid state devices and gas discharge tubes are described in U.S. Pat. Nos. 4,907,120 to Kaczmarek et al. and 5,721,663 to Smith et al.
Another example of an overvoltage protection circuit is provided in U.S. Pat. No. 4,758,920 to McCartney which includes capacitors in the coupling circuit between the gas discharge protection device and the solid state protection devices. A further approach to coupling the gas discharge and solid state protection components is illustrated in U.S. Pat. No. 5,513,059 to Atkins. The Atkins arrangement includes a positive temperature coefficient resistor and inductor and a capacitor in series connecting respective ones of the tip and ring lines between the gas discharge tube protector and the solid state devices. A further variant on the Atkins configuration is found in the TelTect.TM. line protector available from Raychem Corporation which includes a protection circuit like that shown in FIG. 1 of the Atkins patent and further includes an additional solid state voltage clamping diode placed across the tip and ring line rather than between the respective input lines and the line protector ground. The additional voltage clamp device is intended to address the potential problem of a differential voltage spike across the tip and ring lines resulting from transient conditions when one side of the gas discharge tube is switched while the other has not yet switched. Such asymmetric switching may transmit the overvoltage event, which is typically received as a linear voltage spike traveling on both the tip and ring line, into a differential voltage event.
Each of these prior art approaches has various limitations. All the various added components of these protection circuits may provide benefits under the conditions for which they are provided but they often create additional limitations on the circuit. For example, balance should be achieved to provide for the proper coordination between the overvoltage arrestor and the solid state voltage clamping devices, particularly when they are intended to operate in a hybrid manner to provide fast response time rather than merely being provided for purposes of back-up protection. These various approaches further may fail to provide protection under certain conditions such as where an overvoltage condition originates within the line protector device on the equipment side of the protection circuit or where a ground potential differential occurs between the line protector and the communications circuit which is being protected which may be located remote from the line protector itself.